The present invention relates to improved methods and apparatus for transmitting information via a communication link. More importantly, the present invention relates to improved protocols, methods, and devices for transmitting both ATM and packet data over a transport-layer protocol such as SONET in a manner that facilitates more efficient dynamic allocation of bandwidth and traffic management.
The use of the SONET (Synchronous Optical Network) as a transport mechanism at the transport-layer for both (Asynchronous Transfer Mode) ATM and packet data is well known. As the term is employed herein, data includes any type of information that may be represented digitally, and includes such time-sensitive data such as streaming video or voice, and/or non time-sensitive data such as computer files. Packet technology includes TCP/IP, token ring, etc. One example of packet technology is IP (Internet Protocol) packets transmitted over OSI layer 2. Another example of packet technology is Ethernet. ATM and SONET technologies are well known and well defined and will not be elaborated further here. Similarly, the various layers of the OSI 7-layer model are also well known and well defined.
Typically speaking, ATM traffic or packet traffic do not mix in the same unchannelized optical fiber, or in the same TDM channel if the optical fiber is channelized. In the former case, one may think of the entire fiber as a single channel, which is employed to transport either ATM traffic or packet traffic. To facilitate discussion, FIG. 1 is a prior art logical depiction of ATM cells transported within SONET frames in an arrangement commonly known as ATM-over-SONET in an unchannelized optical fiber. As can be seen in FIG. 1, a plurality of ATM cells (53 bytes each) may be packed into a single SONET frame (such as in between SONET overhead blocks 102 and 104). Although only four full ATM cells 106, 108, 110, and 114 are shown, one skilled in the art will appreciate that a typical SONET payload may have the capacity to carry many cells. If there is no room left in a SONET frame to put an entire cell therein, the SONET circuitry may break up a cell and transport a partial cell in that SONET frame (as shown with partial cell 112a). The remaining portion of the cell that was broken up is then transported in the next SONET frame (as shown with partial cell 112b). Furthermore, if there are no ATM cells to transport in a SONET frame, idle cells are typically inserted to fill the frame. Note that since SONET operates at a lower level, as far as the ATM layer is concerned, the fact that an ATM cell was broken up and reassembled by the SONET layer is completely transparent.
FIG. 2 is a prior art logical depiction of packets transported within SONET frames in an arrangement commonly known as packet-over-SONET in an unchannelized optical fiber. Similar to the situation of FIG. 1, a plurality of packets (which may be variable in length) may be packed into a single SONET frame (such as in between SONET overhead blocks 202 and 204). However, since the packets may have variable lengths, flags are employed between packets to help delineate where the packets are in the data stream. With reference to FIG. 2, these flags are shown as flags 206, 208, 210, 212, and 214. Flag 214 is a flag inserted to fill in the frame if there is no packet data to fill. Again, if the frame is full, a packet may be broken up to be transported in different frames. This is shown with packet 216, which is shown broken up into partial packets 216a and 216b and transported in two different frames. Again, since SONET operates at a lower level, as far as the packet layer is concerned, the fact that a packet was broken up and reassembled by the SONET layer is completely transparent.
In either case, the entire unchannelized optical fiber is used to transport only ATM traffic (FIG. 1) or packet traffic (FIG. 2). To allow ATM traffic and packet traffic to share a single optical fiber, the optical fiber may be channelized into different time division multiplex (TDM) channels. Within each channel, the traffic is again either entirely ATM or entirely packet. This situation is logically depicted in FIG. 3.
In FIG. 3, an STS-3 SONET transport arrangement is shown, wherein the full bandwidth of the optical fiber is channelized into three different STS-1 channels or time slots: slots 1, 2, and 3. ATM cells are shown packed into slot 1, while packets are shown packed into slot 3 to illustrate that ATM and packet traffic may occupy different channels to share the optical fiber. Again, both ATM cells and packets may be broken up for transport in parts if the bandwidth available in a slot is already full.
Prior art FIG. 4 shows, in a high level depiction, how ATM and packet data from various sources may be multiplexed into different separate channels in the same optical fiber. With reference to FIG. 4, ATM data is destined for slot 1, while packet data is destined for slots 2 and 3. A multiplexer 402 takes data from the ATM stream 404, the packet stream 406, and the packet stream 408, and multiplexes them using a time-division multiplexing scheme onto time slots 1, 2 and 3 respectively. At the other end, a demultiplexer 410 demultiplexes the data into ATM stream 412, packet stream 414, and packet stream 416, respectively.
While the TDM multiplexing scheme of FIGS. 3 and 4 allow ATM traffic and packet traffic to share different separate channels in the same optical fiber, there are disadvantages. Byway of example, within each STS-1 channel (which is 51.84 Mbps), the traffic within each channel must he either all ATM cells or all packets. Because of this limitation, the channels must be pre-allocated in advance for each type of traffic. If a given type of high-priority traffic (e.g., streaming video over ATM) is assigned to a given channel or time slot, and the bandwidth requirement associated that traffic type increases beyond the capacity of the channel, transmission delay can occur. Even though the network operator can assign one or more additional channels to handle the increase in this ATM traffic, there is a nontrivial time delay associated with the detecting the congestion condition (and possibly concurrently coordinating the unallocation of channels previously allocated to other non-priority traffic if there are no free channels left), coordinating with the receiving end to allocate the additional channels, and changing network parameters to allocate the additional channels to handle the increased bandwidth requirement of the high-priority traffic. As can be appreciated from the foregoing, the complex coordination, unallocation of channels and reallocation of them to the priority traffic involves a nontrivial amount of operational complexity and/or time delay. For some types of data (e.g., time-critical data), this delay is unacceptable. For this reason, many network operators tend to reserve an unduly large amount of bandwidth overcapacity to handle potential traffic increases when dealing with time-critical data, which leads to inefficient use of network bandwidth as the reserved bandwidth tends to stay unused most of the time. Furthermore, each traffic type can employ bandwidth only in a discrete, channel-sized chunk. If additional bandwidth is allocated to a given traffic type, an entire slot (51.84 Mbits in case of STS-1) is added even if the increase in traffic only requires a portion of the capacity offered by an entire slot.
In view of the foregoing, there are desired improved techniques for allowing ATM traffic and packet traffic to share the bandwidth of a channel in a manner that facilitates dynamic allocation of bandwidth between traffic types and more efficient traffic management.